Sensors such as a Shack-Hartmann wavefront sensor are often used to characterize the spatial characteristics of optical wavefronts, including the spatial characteristics of laser beams. Sensors of this type employing prior art may include a charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) focal plane array (FPA), or other type of FPA, to detect optical radiation, such as laser radiation. In such sensors, a microlens array is placed in front of the CCD, CMOS, or other image sensor, and the wavefront slope at a number of transverse locations of the wavefront is determined by measuring the location of the centroid of the focal spots formed by each lenslet of the microlens array.
Although the prior art wavefront sensors, including Shack-Hartmann wavefront sensors, allow for extensive and useful characterization of optical beams, these sensors are limited in their wavefront sensing capability due to limitations imposed by two major factors. The first factor is the limitation on spatial resolution of the microlens array used in these sensors. The second factor is the limitation on angular resolution imposed by the focal length of the microlenses composing the microlens array, combined with the pitch of the CCD, CMOS, or other image sensor used to detect the optical beam.
The spatial resolution of a wavefront sensor constructed using the prior art is determined by the pitch of the microlens array, that is, the distance between centerlines of adjacent microlenses of the array. The spatial resolution cannot be indefinitely increased by the manufacturer because reducing the pitch (thus obtaining a higher spatial resolution) while keeping the microlens focal length constant also reduces the range of wavefront slopes measureable by the wavefront sensor.
The angular resolution of a wavefront sensor of the Shack-Hartmann type is determined by the focal length of the individual microlenses of the microlens array and the pitch of the CCD, CMOS, or other image sensor used to detect the focused spots produced by the microlens array. The angular resolution of the wavefront sensor cannot be indefinitely increased by the manufacturer by increasing the focal length of the microlenses because increasing the focal length (thus obtaining greater angular resolution) while keeping the pitch of the microlens array constant also reduces the range of wavefront slopes measureable by the wavefront sensor.
For any given wavefront sensing task, there will typically be an optimum combination of values of the pitch and focal length of the microlens array, such that the wavefront sensor is capable of measuring the full range of wavefront tilts that are present in the measured wavefront, and also provides adequate spatial resolution. A fixed microlens array with fixed pitch and fixed focal length will not be optimal for every wavefront measurement task encountered by users.
One method that has been used in prior art to allow optimization of a Shack-Hartmann wavefront sensor for each particular wavefront sensing task is the provision of field-replaceable microlens arrays. With this method, the user picks a microlens array for each wavefront measurement task, and installs it into the wavefront sensor. Typically, a lengthy and difficult calibration procedure must be performed each time the microlens array is removed and replaced. The process of removing and replacing the microlens array thus becomes time-consuming and expensive.
A Shack-Hartmann wavefront sensor constructed using prior art provides measurements of the beam power density with a spatial sampling period equal to the period of the microlens array. For measurements in which it would be valuable to measure the beam power density with a period smaller than the period of the microlens array, the microlens array could be removed, thereby offering the opportunity to measure the beam power density with a sampling period equal to the pitch of the CCD, CMOS, or other image sensor. However, as noted above, removing and replacing the microlens array is, if it is possible at all, a time-consuming and expensive process.
Thus, there is a need for a wavefront sensor that can be optimized by the user, or automatically by the wavefront sensor system, without the need for removal and replacement of the microlens array.